1. Field of the Invention
The present invention relates to a track zero cross signal generation apparatus for use in an optical disk system, and more particularly, to an apparatus for generating an accurate track zero cross signal, by correcting a tracking error signal acquired when an optical pickup traverses tracks on a recording medium.
2. Description of the Related Art
In a general optical disk system for recording and reproducing data, a tracking error signal (TES) is used as a tracking servo signal for an optical pickup device in order to accurately focus a beam spot on the center of a track. The optical disk system performs a seek servo control to cross tracks for high-speed seek when a target track is far from a current track. In this case, an optical pickup in the optical disk system traverses the tracks in the radial direction on an optical disk such as a compact disk (CD) or a digital versatile disk (DVD). At the same time, a photodetector in the optical disk system detects light reflected from the recording surface of the optical disk to thereby generate a tracking error signal. The tracking error signal is input to a comparator which functions as a track zero cross signal generation apparatus. The comparator compares the input tracking error signal with predetermined reference voltage levels, and generates a track zero cross signal (TZCS) based on the comparison result. A seek servo apparatus counts the number of rising edges or falling edges of the generated track zero cross signal, calculates the number of the traversed tracks, and moves an optical pickup from a current track to a target track based on the calculated number of the traversed tracks.
Meanwhile, it is more critical in case of a DVD to access to a target track at a high speed, than in a CD. Thus, in the case of a DVD system, a track zero cross signal should be accurately made from a tracking error signal obtained when traversing tracks at high speed. By the way, a track zero cross signal may be improperly generated from a faulty tracking error signal with noise in its high frequency band. Therefore, a general optical disk system performs low-pass filtering of the above-described tracking error signal in order to remove the noise. However, such a low-pass-filtered tracking error signal may cause a phenomenon where an envelope of a predetermined high-frequency band in a pass band becomes smaller. Also, when an optical pickup device works at a high speed, the optical axis of an object lens tends to be aligned differently from arrangement of the optical axis when the optical pickup device operates at a normal speed. Accordingly, the reflected light passing through the center of the object lens after being reflected from the optical disk may be detected in a position departed from the center of the photodetector. For this reason, the center level of the tracking error signal obtained from the optical disk differs from the zero level which is the detection center level of the photodetector.
FIG. 1 is a graphical state diagram showing a phenomenon where a tracking error signal obtained when an optical pickup traverses tracks on a recording medium at a high speed is distorted. In FIG. 1, an X-axis represents time and a Y-axis represents a voltage level. A reference character E represents an envelop of a tracking error signal. HF represents a relatively high-frequency tracking error signal. LF represents a low-frequency tracking error signal. Z represents a zero level of a photodetector. C represents the center level of the reflected light passing through an object lens, which depicts that the reflected light detected by the photodetector is off from the zero level by a level of -.DELTA.V. In the case of the tracking error signal of FIG. 1, an envelop of a relatively high-frequency band is smaller than that of a low-frequency band, and the center level of a tracking error signal is off the zero level. The tracking error signal is generated mainly due to a low-pass filtering and an off-axis of the object lens. In addition to the above main causes, the tracking error signal as shown in FIG. 1 may always be generated since various external disturbances occur due to a high-speed operation of the optical pickup device and a high-speed rotation of the optical disk.
A conventional track zero cross signal generation apparatus receives a tracking error signal described with reference to FIG. 1 and generates a track zero cross signal. In more detail, the tracking error signal obtained from the optical disk is input to a comparator during seek servo control. The comparator compares the input signal voltage with predetermined reference voltage levels, and then generates a track zero cross signal having mutually different pulse levels whenever a voltage level of the input signal is larger than a first reference voltage level or whenever a voltage level of the input signal is smaller than a second reference voltage level. Here, the comparator accurately generates a track zero cross signal having different pulse levels only when the center level of the tracking error signal input to the comparator coincides with the zero level relating to the predetermined reference voltage levels, and the envelop is larger than the first reference voltage level or smaller than the second reference voltage level. Thus, when the tracking error signal input to the comparator is distorted as in the FIG. 1 tracking error signal irrespective of whether low-pass filtering is performed, the comparator misses the track zero cross signal at a predetermined high-frequency band. Accordingly, a conventional optical disk system which fails to generate a track zero cross signal accurately cannot access a target track accurately and quickly.